drm/amdgpu: add vram check function for GMC

drm/amdgpu: add vram check function for GMC

This patch will add vram check function for GMC block.
It will write pattern data to the vram and then read back from the vram,
so that to verify the work status of vram.
This patch  will cover gmc v6/7/8/9/10.

Signed-off-by: Xiaojian Du <Xiaojian.Du@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>